Dental handpieces operated by air turbines have a turbine rotor that is supported in a housing a sliding bearing or a ball bearing. The turbine rotor has turbine blades at its circumference as well as a clamping device for clamping the shank of a dental tool within a housing, the turbine blades being mounted on the outside surface of this housing. In order to fit and change the tool, push-button clamping systems are known that do not require an extra tool for a tool change. Two popular designs for push-button clamping systems have gained acceptance. In one design, a taper is used in conjunction with a pressure spring. The other design is based on the elastic deformation of a specially constructed component that is shown, for example, in EP 1 232 731 B1.
A known clamping device that is described in CH 631 068 A has a chuck having a plurality of tensioning claws and a clamping piece that interacts with the tensioning claws. The clamping piece clamps the tensioning claws from the outside towards the inside in the direction of the tool shank, so that the tensioning claws concentrically exert a clamping force on the tool shank. The shank of the dental tool is thereby securely clamped. However, the difficulty arising here is that when the turbine rotor is at a standstill, the shank of the dental tool continues to be securely held in the clamping device, whereas the holding force during operation decreases due to the centrifugal forces acting on the tensioning claws. Moreover, in this design it is necessary to operate the clamping device manually when fitting and releasing the dental tool.
CH 631 068 A further describes that likewise when fitting and removing a dental tool by means of elastic clamping using a plastic or rubber element, there is the difficulty that centrifugal forces act on the elastic clamping device and again during operation the holding force thus decreases in a centripetal direction.
CH 631 068 A therefore proposes an air turbine handpiece having a clamping piece within the turbine rotor body in which the chuck is fitted and arranged so as to be slideable in an axial direction. On its front part, the chuck has a large number of tensioning claws, and on its opposed rear end there is located a flange which supports an elastic ring. When the turbine rotor body rotates, the centrifugal force spreads the ring apart and thus presses it against the tensioning sleeve which is advanced in the clamping piece. The tensioning claws are thereby clamped together convergently, so that the shank of the dental tool remains securely clamped even at increasing rotational speeds.
US 2002/0105149 A1 also deals with the problem of a decreasing clamping force as the rotational speed of the dental turbine increases, due to the increasing centrifugal force. This document proposes a solution for a clamping device of a dental tool having a housing in which a drill sleeve is inserted. The aim of the design is to allow easy insertion of a dental drill when the dental turbine is at a standstill and to lock it in place using the centrifugal forces that act during rotation. For this purpose, locking weights are provided within the housing that are supported at associated fulcrums and, on rotation of the dental turbine, pivot in a radial direction about their fulcrums and come to rest against the shank of the dental drill in order to lock it in position. The weights pivoting in a radial direction jut out and can only be disposed near the turbine head. It is not possible to accommodate them within the inside diameter of the turbine blade to save space.
The systems described achieve a certain improvement with regard to the problems caused by centrifugal forces occurring on rotation. In practice, however, the solutions have not proved very useful.
The basic requirement placed on a clamping device for a dental tool in a dental turbine handpiece is that it should be possible to change the dental tool with the least possible actuating force and without the need for extra tools. This actuating force, however, must not be made so small that unintended triggering, such as by touching the cheek of a patient, is made possible. The holding force must be sufficient to lock in the dental tool reliably and securely under all operating conditions. A desirable clamping system is thus one that is simple and fast to operate and that has sufficient holding force to clamp a dental tool reliably. Since nowadays a cylindrical shank is always standard for dental tools, it is not possible to have form-locking clamping of the shank of the dental tool in order to secure the dental tool against axial displacement and against rotation. Rather, it is necessary to hold the dental tool in position using a frictional connection and the resulting frictional forces. Here, the clamping device has to be designed such that, even for centrifugal forces acting at very high rotational speeds from up to 450,000 rpm and more, it holds the dental tool reliably, since even dental handpieces having ball bearings nowadays reach rotational speeds of up to 450,000 rpm. A solution is sought that makes use of these centrifugal forces for clamping the dental tool in a simple manner so that on rotation of the dental turbine, the holding force definitely does not decrease in relation to a stationary system.